Intelligence Ink Detects Oxygen

An ultraviolet-light-activated ink formulation developed by chemists in Scotland reveals the presence or absence of oxygen inside packages. The ink is blue in air and ambient room light. When it is irradiated with a pulse of UV light, the color changes to white but reverts to blue under normal room light. In an oxygen-free atmosphere, however, the ink remains colorless after the UV pulse [Chem. Commun., published online July 9, http://xlink.rsc.org/?doi=10.1039/b406685e].

"The ink is an 'intelligence ink,' as it provides intelligence--that is, information--about its environment," Mills explains. "Intelligence inks are at the cutting edge of packaging, especially of foods and beverages, since they can indicate if a package has been packed properly or tampered with or if the original modified atmosphere inside the package has changed."

Modified-atmosphere packaging is used extensively in the food industry. Typically, air is removed from the space above the food as it is being packed by flushing the package with an inert gas such as carbon dioxide or nitrogen.

"Our ink can be readily used with packaging," Mills says. "The color change means that even an untrained eye can decide if a package is faulty."

The formulation consists of four common and inexpensive ingredients: an aqueous dispersion of a semiconductor (TiO₂), a sacrificial electron donor (triethanolamine), and aqueous solutions of a redox-indicator dye (methylene blue) and an encapsulating polymer (hydroxyethylcellulose).

The TiO₂ particles create electron-hole pairs when exposed to UV light. The electrons reduce the dye, causing it to be bleached, and the holes oxidize the triethanolamine. Polymer encapsulation allows the dye to be spin-coated onto plastic, metal, paper, or other surfaces.

"The ink can be reused by repeated application of UV light," Mills says. "It is just one of many we have produced for [sensing] oxygen. A variety of different color changes and sensitivities are possible."

The authors note that before the ink is used in the food industry, a key question will have to be addressed: Do antimicrobial or antioxidant food additives or microbial contaminants interfere with the indicator's function?

The Strathclyde team is now carrying out research to answer that question. "We are also developing inks for other gases, such as ammonia and CO₂," Mills says.